This invention relates to a novel phenolic resin, a method for producing the resin, and an epoxy resin composition for encapsulation containing the phenolic resin as an essential component, wherein the composition is superior in mechanical and electrical characteristics, moisture proofness, thermal resistance, impact resistance and resistance to cracking even when subjected to severe temperature fluctuations.
Recently, in keeping up with the rapid progress in scientific technology centered around the electronic industry, a more and more stringent demand is placed on the properties of products and the starting materials. Above all, in the field of semiconductors the degree of integration is becoming higher rapidly and, in keeping pace therewith, the interconnection is becoming finer and the chip is becoming larger in size. On the other hand, as the degree of integration of semiconductor devices becomes higher, surface mounting tends to be preferred rather than through-hole mounting. In an automation line of surface mounting, semiconductor packages tend to suffer acute temperature changes during soldering of lead lines to produce cracks in molding resins or the resin interface region between the lead lines is deteriorated to lower moisture proofness.
For overcoming the above problems, the methods of adding silicone compounds or thermoplastic oligomers or of silicone modification have been proposed with a view to releasing thermal impact at the time of immersing semiconductor packages in a soldering bath. However, with these methods, resin compositions for encapsulating semiconductors which are reliable in use have not been produced because cracks are developed in the molded products after immersion in the soldering bath.
In the resin compositions for encapsulating semiconductors, phenolic resins are used as a curing agent for epoxy resins. As these phenolic resins, novolak phenolic resins or novolak cresol resins are employed. However, the phenolic resins suffer from a drawback that the semiconductor packages exhibit strong hygroscopic properties, so that cracks are inevitably produced during immersion in the soldering bath, as mentioned previously. Recently, a proposal has been made for improving the phenolic resin, i.e. a curing agent for the epoxy resin composition to improve thermal resistance of the semiconductor encapsulating resin compositions. For example, a dicyclopentadiene-modified phenolic resin is proposed as a heat-resistant phenolic resin exhibiting superior moisture proofness in Japanese Laid-open Patent Application No. 63-110213 (1988). However, the dicyclopentadiene-modified resins are low in reactivity and inferior in moldability, while exhibiting a high softening temperature. Phenolic resins derived from norbornadiene, cyclopentadiene or cyclohexadiene as starting materials have also been proposed in Japanese Laid-open Patent Application No. 2-187422 (1990). However, the semiconductor encapsulating resin compositions employing these phenolic resins present problems in respect of hygroscopic properties such that it is not possible to prevent cracking completely during immersion in the soldering bath. As for the manufacture, with the above mentioned particular phenolic resins derived from dienes such as dicyclopentadiene, norbornadiene, cyclopentadiene or cyclohexadiene, a labor consuming operation is necessitated in removing unreacted feed materials or by-produced polymers of the above mentioned dienes, while the produced phenolic resins tend to be costly by reason of the above-mentioned manufacture problems.